The efforts of the Molecular Mechanisms of Tumor Promotion Section are directed toward understanding the ear?y events in the interaction of phorbol ester tumor promoters with cells and tissues. Particular attention is being devoted to the analysis of the major phorbol ester receptor, protein kinase C. The bryostatins, macrocytic lactones, activate protein kinase C and compete for phorbol ester binding. They only induce a subset of the typical phorbol ester responses, however. In Friend erythroleukemia cells, they restore differentiation inhibited by the phorbol esters. In primary mouse epidermal cells, they induce markers of the proliferative response but block phorbol ester induction of markers of differentiation. Part of the difference in response pattern can be explained by the bryostatins functioning to activate protein kinase C transiently, followed by suppression of the pathway. Thus, both for cell-cell communication and epidermal growth factor binding, the bryostatins initially act like the phorbol esters but subsequently block phorbol ester responsiveness. In addition, the bryostatins intrinsically differ from the phorbol esters in their stimulatory activity for some responses; for example, they fail to induce arachidonic acid release in C3H10T1/2 cells even at very early times. The biochemical mechanisms for these differences are being explored through immunoblotting of protein kinase C, comparison of phosphorylation patterns, tritiated bryostatin binding analysis, and comparison of effects on cloned and chromatographically separated protein kinase C isozymes. Structure-activity analysis suggests that bryostatin derivatives differ in the degree to which they are bryostatin-like in their actions rather than phorbol-ester like. Computer modeling indicates excellent fit to the previously derived phorbol ester pharmacophore and is consistent with the structure-activity relations.